The SFO said last year it had "been approached by a number of sources who may have information relevant to this investigation."

Unaoil has denied the allegations of corruption.

Petrofac said in a statement released Friday that the Serious Fraud Office has notified the company about the investigation.

"The Company believes that this is in connection with the investigation into Unaoil," Petrofac said.

The statement also said,

As previously disclosed, Petrofac engaged Unaoil, a Monaco based company, for the provision of local consultancy services primarily in Kazakhstan between 2002 and 2009. The Company is cooperating with the authorities. Ayman Asfari, Chief Executive Officer, and Marwan Chedid, Chief Operating Officer, have been questioned under caution by the SFO.